Well, I had a productive day today with my in-Law's Mac Mini as the test subject. The 32-bit app I was trying to move away from was the 2008 version of Entourage, Microsoft's Outlook client for OS X that came out as part of the Microsoft Office 2008 for Macintosh. So, yes, it's unsupported. The direct upgrade patch is to convert from Entourage to Outlook 2011 on the Mac and then everything is roughly equivalent to the Windows version (.OST & .PST files for the local Mailbox Storage). Since I didn't have an easy (or legal) way to get Office 2011, I figured I was screwed.
What happened today was nothing short of miraculous. In Entourage, one of the tools that I had was to Export the Account Profile and create a .RGE file. This is where I was coming up empty on my previous queries, how to go from a .RGE file into a .PST file, or some form that I could transfer over to the native Mail App on macOS. I figured I would convert everything to Mail since it would be like the Mail app on the iPad. But there was this .REG file to deal with, or somehow create a private IMAP or POP server that I could use to forward all the email messages into and then bring them over to Mail.
One of the niftier features of the Macintosh's operating environment is they way Applications are packaged. It's like the Mac Apps are Zip Archives, and through the "Show Package Contents" command, you can explore the App's resources through a hierarchical directory structure. So I right-mouse click on the .RGE file, pick "Show Package Contents", and see the Mailbox broken into Inbox, Sent, Drafts, Junk, and Deleted folders, each with its own .MBOX file. Well, come to find out .MBOX is a Unix mailbox format, and it's pretty much supported by most every Mail application out there, including the macOS Mail App.
I made a copy of the .MBOX files in a directory off of the user root. I was then easily able to Import these .MBOX files and all the messages included in them to the Mac's Mail App, under "On the Mac". Once I connected the user email accounts to Mail, I could easily drag the Imported messages into the different Mailboxes and my Father-in-Law was in business using Mail. Now we can upgrade to Catalina on the Mac Mini. Just need to figure out the other two lynchpins in the Office Suite, Word and Excel. He's not crazy about paying $70/yr for a single user license of Office 365.
Oh, and the kicker is after I get all this figured out today and as I'm typing this post, I find this:
Extract Entourage RGE File on Mac & Windows OS which basically spells out everything that I needed to do, but from 4-5 years ago.